Mitchell Cruzan is Professor of Evolutionary Biology at Portland State University. For Fascination of Plants Day on 18 May this year, he talked to us about his research and shared some fascinating insights into the evolved adaptations that distinguish plants from animals.
Your research looks at how plants differ from animals. Can you tell us what the key such differences are?
There are two fundamental differences between plants and animals. The first is that plants are sedentary. Since all of the creatures that we share an affinity with have some form of mobility, not being able to move might seem like a limitation. In reality, it’s simply an outcome of their evolutionary history; plants don’t move around because they don’t need to. Early in the history of life on Earth, when only microbes were present, some lineages of bacteria acquired the ability to convert light energy into chemical energy, known as photoautotrophism. This was a completely novel mode of living; the new photoautotrophs just had to sit still and absorb light, carbon dioxide, and nutrients to make their own food. This changed everything; now, there was an abundance of food energy and oxygen for efficient metabolism. Photoautotrophs enabled the evolution and diversification of all life that followed and is responsible for the incredible biodiversity we find around us today.
“all of the chardonnay vines grown around the world are parts of a single massive clone”
The second major way that plants differ from animals is that they can keep growing into adulthood; they have indeterminant growth. They do this by regenerating themselves as they elongate their stems and produce new leaves and buds. Even for very old trees, the new growth at the tip of each stem looks as fresh and new as when they were seedlings. In contrast, animals stop growing when they become adults, and it’s downhill from there as senescence eventually leads to death. The regular regeneration of new tissue by perennial plants allows them to be effectively immortal, and death only comes with disease or destruction. Humans have taken advantage of this unique characteristic as many of the plants we rely on for food production, such as fruit trees and grape vines, are propagated with cuttings. Even though cuttings are grown as separate plants, they are still part of one individual plant that originated from a single seed. For example, all of the chardonnay vines grown around the world are parts of a single massive clone.
Since plants can’t move around, does that put them at a disadvantage as compared to animals in adapting to changing environments?
It might seem that way, but it turns out that plants may be able to adapt to variations in their environments more easily than animals. They do this in three ways. First, plants can change as they grow. This is known as phenotypic plasticity. Plants modify their leaves as they develop so they are suited to the current environment. For example, leaves developing in the shade are larger and thinner, and plants exposed to drought will produce smaller leaves that may be covered with hairs. Phenotypic plasticity is typically due to reversible epigenetic changes—the addition of proteins to the DNA molecule that control gene expression. Such epigenetic modifications can allow plants to adjust to changes in the environment over a matter of minutes as they grow new leaves and stems. Indeterminant growth and a high degree of plasticity allow plants to continue thriving while they remain sedentary and are exposed to seasonal and longer-term changes in their environments.
Second, plants can pre-condition their seedlings to improve their chances of survival. The epigenetic changes made during a plant’s development can be inherited by their seedlings. In animals, epigenetic changes are removed during gamete formation and have to be reformed during embryo development. The inheritance of epigenetic changes allows plants to prepare their offspring for the particular environments they are likely to encounter. For example, plants subjected to shade or drought stress will produce seedlings that have better survival under the same conditions. Epigenetic responses in plants have been conditioned during their evolution, so unlike random mutations in the DNA code, epigenetic changes are directed and predictable responses to the environment. Once the stress is removed, these modifications are erased until the same stress is encountered in a future generation.
Third, plants can evolve as they grow through clonal evolution. Mistakes in DNA replication—mutations—happen every time cells divide. In animals, heritable mutations only come from cell divisions in specialized cells known as the germ line that form sperm and eggs; mutations in the rest of the body are not passed on to offspring. The situation is quite different in plants, where one group of germ cells at the tip of each stem is responsible for generating the stem and leaves as well as flowers and their reproductive cells. Since plant germ cells undergo many thousands of divisions, the potential for accumulating heritable mutations is very high. Such a high mutational load would be a problem for plants, but clonal evolution during stem growth filters mutations as they arise. If one germ cell acquires a mutation that slows its growth, it will be replaced, and all the mutations it carries will be lost. Mutations that increase growth will be retained as the cells carrying them replace all the other germ cells. By the time flowers are produced, many of the deleterious mutations have been filtered out, while beneficial mutations remain to be passed on to offspring. Viticulturalists have taken advantage of this feature by selecting clonal lines within each grape variety that are characterized by unique sets of mutations that affect their flavor profile and their ability to grow in different climates and soil types.
“We may have fleshy-fruited plants to thank for our own origin, and for our ‘sweet tooth’”
How has the inability of plants to move around affected the way they reproduce?
Since plants cannot move to find mates or disperse their seeds, they rely on biotic vectors, such as bees, and abiotic vectors, such as wind, to accomplish these tasks for them. Plants have undergone selection to manipulate their dispersal vectors to their greatest advantage. For example, plants offer enough floral rewards to entice visits by bees and other pollinators, but not enough to satisfy them, so pollinators are forced to move among flowers and plants to gather enough nectar and pollen. This promotes the dispersal of their pollen and facilitates outbreeding. Many seeds are carried passively by the wind, or by attaching themselves to the fur of animals, but in other cases, plants offer food rewards in exchange for seed dispersal. For example, berries and other fleshy fruits offer a food reward to fruit-eating animals. The seeds in fleshy fruits can pass through the digestive tract without being damaged. As the animal moves around, seeds are dispersed some distance and deposited in new locations along with a bit of fertilizer. We may have fleshy-fruited plants to thank for our own origin, and for our “sweet tooth,” as the earliest primates were fruit eaters that first appeared during the Cretaceous.
Most plants produce both pollen and ovules, so the potential for inbreeding through self-fertilization, also known as selfing, is high. Many flowering plants avoid selfing by a mechanism that arose very much by accident during their origin. The ancestors of flowering plants were fern-like plants that had their ovules exposed on the surface of their leaves so they were susceptible to herbivory by beetles, which became common around the time of the origin of flowering plants in the Triassic. Selection favored the fusion of ovule-bearing leaves to enclose them in an ovary, which protected them from herbivores. One consequence of this new flower structure was the displacement of the location of pollen deposition—the stigma—away from the ovules. Consequently, pollen must grow some distance to fertilize the ovules. This enabled plants to evolve mechanisms to prevent self-fertilization and inbreeding, thereby improving seedling fitness.
This is just a sample of the many ways that plants differ from us. Their sedentary lifestyle, immortal growth, and their ability to change and evolve as they grow may make them seem more alien than any other organism we know. These unique characteristics of plants originated from their autotrophic mode of living and are the outcome of hundreds of millions of years of evolution. Starting with the earliest terrestrial algae, plants have diverged on their own evolutionary trajectories to occupy nearly every habitable environment and produce the amazing diversity of plant life that we enjoy today.
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